Urban and suburban areas of large cities impose great challenges to geophysical surveys because of high-level ambient noise. This is particularly true for controlled source audio-frequency magnetotellurics (CSAMT) because the frequency range of the method overlaps those of many civil and industrial noises. Among the different types of noise sources, perhaps the most noticeable one is the 50-Hz power grid. We have successfully applied the CSAMT method for groundwater exploration in a suburban Beijing area where the ambient noise level is about three times as high as would normally be encountered. Several steps were taken in assuring quality use of the method, including careful survey design, adequate frequency range selection, and large transmitter dipole length and transmitter-receiver spacing. The next step was static effect removal through low-pass filtering and topography correction to remove any nonhorizontal component in the electric field measurement. Finally, a 1D inversion method was applied to construct an (apparent) resistivity cross section. The survey revealed a low-resistivity, water-bearing layer sandwiched between a surface cover layer and the bedrock. The available well data suggested that the depth of the bedrock was accurately mapped, to within about 2 m, at a depth level of 150–165 m.

Detecting and mapping subsurface utilities in urban areas is crucial for identifying defects or damages in drinking and sewage pipes that can cause leaks. These leaks make it difficult to accurately characterize the pipes due to changes in their reflective properties. This study focused on detecting leaks originating from underground pipes and distinguishing between these various types of pipes. It also aimed to create a visual fingerprint model that displays the reflection characteristics of these pipes during different leak conditions, enabling efficient maintenance and handling procedures on the pipes. To achieve this, a finite-difference time-domain (FDTD) method was used to simulate two types of pipe materials with and without leak areas to construct different scenarios. Additionally, a ground-penetrating radar (GPR) field survey was conducted using a 600 MHz antenna in a part of the El Hammam area on Egypt’s northwest coast. The simulated images produced with numerical modeling were compared with the radar profiles obtained using GPR at particular locations. The numerical simulations and radar profiles demonstrated the noticeable influence of water leaks from the different pipes, wherein the reflection of saturated soil waves was interrupted due to the presence of saturated soil. Envelope and migration techniques were employed in a new application to accurately distinguish between different pipe types, specifically focusing on leak areas. The strong correlation between the real radar profile and the specific signal of a water pipe leak in the simulated models suggests that GPR is a reliable non-destructive geophysical method for detecting water pipe leaks and distinguishing between the different pipe materials in various field conditions. The simulated models, which serve as image-matching fingerprints to identify and map water pipe leaks, help us to comprehend reality better.

We use a numerical model to study the effects of attenuation and dispersion upon the migration of ground-penetrating radar (GPR) profiles. A finite-element method (FEM) is developed that incorporates attenuation and is used to generate synthetic GPR profiles with random noise. These profiles are then migrated with and without the attenuation term using our FEM codes. The misfit between the position of interfaces in the model and the position of corresponding interfaces in the migrated profile is greatly decreased when the attenuation term is considered in the migration process. The improvement in resolution results from the use of the group velocity rather than any phase velocity. Consequently, for dispersive media the attenuation term of high-frequency GPR waves cannot be ignored in migration.

Transient electromagnetic method (TEM) is an important tool for mapping the subsurface resistivity. As the electromagnetic fields with different frequencies reach different depth of the Earth, the resolution of TEM method is closely related to the spectrum properties of its source signal. The pseudorandom binary sequence (PRBS) is a recently applied source signal and is thought to be superior to the conventional square wave for its adjustable frequency range and density. However, to invert the response from this optimized source, the pure secondary field needs to be extracted from the full waveform response in which parts of useful information is lost inevitably. We thus propose to directly invert the full waveform TEM response from the excitation of PRBS using a transdimensional framework. We use the reverse-jump Markov chain Monte Carlo (RJ-MCMC) sampler to get the model ensemble so that the posterior of the model can be obtained. The PRBS source is convolved with the impulse response of the Earth to fit the recorded full waveform response. The 2-D model space is discretized using a two-stage binary tree and updates in parallel, which dramatically improve the sampling efficiency. The synthetic examples compare the results from the conventional time-domain step response and the full waveform response and demonstrate that the proposed method obtains higher resolution and can better resolve the interface in the shallow and moderate depth. Furthermore, the real data example recovers the model structure that is consistent with the geologic inference, providing alternate evidence for the conjecture of the Sanshandao fault.